Sunrise Noctilucent Clouds

Sunrise noctilucent clouds (NLC) over Szubin, Poland on July 10, 2011.
July 2011 has been a banner month for these mysterious clouds. Normally confined to polar latitudes, NLCs have been sighted in recent nights as far south as France in Europe and Kansas and Colorado in the United States. In recent years, the NASA AIM (Aeronomy of Ice in the Mesosphere) spacecraft, which was launched to study the NLC phenomenon, has spotted the clouds appearing ever lower in latitudes, but just why is not yet known.
NLCs are seasonal, appearing most often in late spring and summer. In the northern hemisphere, the best time to look would be between mid-May and the end of August.
NLC Observing Tips: Look west 30 to 60 minutes after sunset when the Sun has dipped 6-16 degrees below the horizon. If you see luminous blue-white tendrils spreading across the sky, you've probably spotted a noctilucent cloud. As this photo shows, they can also be seen just before sunrise.
Credit: NASA/Marek Nikodem